The present invention relates to fuel injection control system and method for an internal combustion engine having an odd number of cylinders, and more particularly to such control system and method wherein fuel injection is effected twice in each engine cycle on the basis of ignition signals.
A system for controlling fuel injection into an internal combustion engine is well known in which an electronically-controlled fuel injection system wherein the fuel injection quantity to be supplied to the engine is controlled by the pulse duration of a fuel injection control pulse signal applied to solenoid-operated injection valves to meet power demands under varying engine operating conditions and fuel injection is effected twice at intervals of two revolutions of the engine crankshaft or each engine cycle in synchronism with an ignition signal generated by an ignition system of the engine.
Such a system is being used successfully for internal combustion engines having an even number of cylinders, such as four cylinders. However, it is not compatible with internal combustion engines having an odd number of cylinders because it is impossible to obtain equally spaced fuel injection timings when it is intended to perform fuel injection in twice in each engine cycle on the basis of the ignition signal. More specifically, if the ignition signal is used as it is for causing fuel injection, fuel injection occurs in a period of time between two successive ignitions.
One possible solution is to provide a sensor for sensing the angular position of the engine at intervals of 180.degree.. However, this results in a complex structure and an increase in cost. U.S. Pat. No. 4,180,023 issued Dec. 25, 1979 to the same asignee as the present invention discloses an electronically-controlled fuel injection system for internal combustion engines having an odd number of cylinders for eliminating the above-mentioned problems. This involves a technique wherein the fuel injection is effected after a elapse of time from an ignition timing by the half of the time interval between successive two ignition timings.
According to this prior art system, however, the fuel injection timing suffers from an undesirable delay when the engine speed increases and also suffers from an undesirable advance when the same decreases. That is, this system makes it difficult to stabilize fuel injection timing under varying engine operating conditions, resulting in the deterioration of exhaust emission and the occurrence of shock within a vehicle.